# Advantage of sequential logic

1. Oct 7, 2012

### Avichal

There are some circuits which can be made only using sequential logic like registers and counters.
But some like adders can be made both by combination logic and sequential logic.
So which one is better in such cases?
I guess sequential logic is advantageous but what advantage does it offer?

2. Oct 8, 2012

### FailedLaunch

The biggest difference between combinatiorial and sequential logic is that sequential one uses memory parts (flip-flops, such as SR, JK, T, D, ...) and besides current values at its inputs can also create output(s) based partly or fully on previous values (memory parts store so-called "states" which are either previous values on its inputs or calculated values). Combinatorial logic doesn't have these memory cells and creates output based purely on its current inputs.

Sequential logic si typically clocked ("synchronized"), so it produces a new guaranteed correct output only in certain times ("with clock").

Adder in combination logic is usually much bigger (many gates/parts, high power consumption) but also much faster. As with any combinatorial logic where you combine many signal paths to come to a result, a varying delay among many sub-branches keep you on toes trying to figure out when the value at the output is correct (generally after the signal propagates through the slowest -- in many cases it is the longest -- branch). The advantage is that the result is available immediately when it's known and one doesn't need to wait till the next clock pulse to be sure that the value is correct.

Asynchronous sequential logic lies between combinatorial and synchronized/synchronous sequential logic - it contains memory cells and remembers its past and can base its outputs on those values, on the other hand it suffers from the same delay/uncertain(incorrect results) (the output can change several times as signal arrives from different sub-branches and only after the slowest arrives is the result correct).

I'm not sure if it answers your question.

Also look at Wikipedia (Sequential logic).

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